Recent studies have shown that blood flow during closed-chest cardiopulmonary resuscitation (CPR) results from cyclic variations in intrathoracic pressure, rather than direct compression of the heart. Chest compression produces an increase in intrathoracic pressure which is transmitted unequally to extrathoracic arteries and veins, thereby creating a driving force for systemic blood flow. However, the fact that intrathoracic arterial and venous pressures rise and fall to similar levels during CPR prevents the development of a comparable driving force across the coronary circulation. Preliminary studies from this laboratory have confirmed this theoretical limitation to coronary flow during CPR. However, coronary flow is not entirely absent. Standard CPR produces a small aortic-right atrial pressure difference and low levels of coronary blood flow, both of which occur primarily during the relaxation phase of each compression cycle. The projects outlined in this application are designed to determine (1) the mechanism responsible for this pressure difference;(2) whether this pressure difference (and therefore coronary blood flow) can be augmented by interventions which either increase artificial cardiac output (such as volume loading) or alter relative arterial abd vebiys resustabce abd caoacutabce (such as alpha-adrenergic stimulation); (3) whether similar augmentation can be accomplished by changes in resuscitationn technique; (4) whether the driving pressure across the coronary circulation can be increased selectively by synchronized intraaortic counterpulsation; (5) whether manual (open-chest) ventricular compression promotes coronary blood flow more effectively than standard, closed-chest CPR; and (6) whether the low levels of coronary flow present during closed-chest CPR are adequate to meet myocardial metabolic demands during ventricular fibrillation. It is hoped that these studies will provide both further insight into the dynamics and metabolic consequences of reduced coronary blood flow during CPR, and an objective assessment of potential methods of improving coronary flow during attempted resuscitation.